(C++) error: invalid conversion from ‘char’ to ‘const char*’ [-fpermissive]

Question

I'm trying to write a function that would return a substring from the given start and end indices. This is the code that I've written but when i run it it gives me error. I'm using gtest to run it not main().

template <typename S>
S substring(S string_, int Istart, int Iend)
{
    S substr = string_[Istart];
    for(int i=(Istart+1); i<Iend; i++)
    {
        substr += string_[i];
    }
    return substr;    
}

And this is the error i get:

In file included from test.cpp:2:0:
lab2.cpp: In instantiation of ‘S substring(S, int, int) [with S = const char*]’:
test.cpp:56:1:   required from here
lab2.cpp:91:23: error: invalid conversion from ‘char’ to ‘const char*’ [-fpermissive]
     S substr = string_[Istart];
                ~~~~~~~^

My test code is this:

TEST(subStr, T1){
string str="he";
EXPECT_EQ(str,substring("hello", 0, 2));
}
TEST(subStr, T2){
string str="urge";
EXPECT_EQ(str,substring("hamburger", 4, 8));
}

Answer 1

Assuming that the intended type is string. A += operator exists for the string type.

A simple fix is to replace

S substr = string_[Istart];

with

S substr;
substr += string_[Istart];

which will not work for const char* types as pointed out

The following works for both string and const char*.

#include <exception>
#include <iostream>
#include <functional>
#include <string>
#include <type_traits>

template <typename S>
S substring(S string_, int Istart, int Iend)
{
    if constexpr (std::is_same_v<S, std::string>) {
        if(Istart < Iend){
            return string_.substr(Istart, Iend-Istart);
        }
    }
    else if constexpr (std::is_same_v<S, const char*>) 
    {
        if(Istart < Iend){
            std::string temp = std::string(string_);
            return (temp.substr(Istart, Iend-Istart)).c_str();
        }
    } 
    else throw;
}